1. Technical Field
The present invention generally relates to the field of electronic measuring devices and more particularly, is directed to a device for measuring the rate of rotation of an object without being adversely affected by environmental conditions, improper alignment and spacing with the object being measured or a change in internal component values due to temperature drift and aging.
2. Related Art
Conventional rotation detectors and measuring devices utilize eddy current probes. Such probes are well known for their ability to detect the presence of ferrous and nonferrous conductors. Such probes generally operate by inducing radio frequency (RF) energy in metals adjacent the sensing probe and then monitoring the excitation voltage at the sensing probe. An example of such a probe is illustrated in FIG. 1 where RF energy from oscillator 20 is induced in metal object 15 by sensing probe 10. The energy induced in object 15 is known as eddy currents 16 and results in a decrease in excitation voltage at sensing probe 10. In a simple switching application where detection of the presence or absence of the object is all that is required, the excitation voltage at sensing probe 10 is rectified and filtered by element 30, amplified by element 80 and compared to a reference value by comparator 90 which provides a corresponding logic detection signal. The on and off logic signal represents absence or presence of the detected object.
An alternative to eddy current probes, such as described above, are magnetic reluctance probes which are used to detect the presence of a ferrous (permeable) object. Such probes are formed of a magnetically biased solenoid coil with an open magnetic structure designed to be influenced by a magnetic object which passes by a fluxed gap. The AC voltage generated in a magnetic reluctance probe is a function of the proximity of the probe to the moving magnetic object, a function of its relative permeability and a function of its velocity relative to the sensing probe. A disadvantage of magnetic reluctance probes is that the magnetic object must be moving for the magnetic reluctance probe to sense its presence. Furthermore, at low relative velocities, the output of a magnetic probe is very low (in the mV range) and can be subject to interference from a variety of external sources, both magnetic and electric.
Photoelectric position sensors also are used as an alternative to eddy current probes. Photoelectric positions sensors operate in either a reflective or transmissive mode; that is, as an optical beam interruptor or as a reflective object sensor. In either embodiment, the relative sensitivity to dirt contamination and limited temperature operating range of photoelectric position sensors precludes their use in most outdoor, industrial or automotive applications.
Hall effect sensors can also be used as an alternative to eddy current probes for certain applications. Hall effect sensors are responsive to a bias field from a magnet. Unlike eddy current sensors a magnetic bias field is required and unlike magnetic reluctance probes the magnetic bias field does not have to be moving to sense its presence. Hall effect sensors would not be considered for use in certain applications because they have disadvantages when used in less refined conditions such as industrial or automotive conditions. Hall effect sensors use a permanent magnet that will pick up metal fillings and clog the stand-off space between the sensor and the object to be sensed. Additionally, Hall effect detectors require a smaller stand-off space than eddy current probes which further compounds problems with metal fillings or dirt which will clog the stand-off space. Also, a smaller stand-off space increases alignment problems beyond tolerable limits. Furthermore, Hall effect sensors are susceptible to temperature drift and are incapable of operating at elevated temperatures.
Only eddy current probes, photoelectric position sensors and Hall effect sensors can be used at very low sensing velocities. Magnetic reluctance probes can not be used at low sensing velocities because the magnetic object must be moving for the magnetic reluctance probe to sense its presence. Eddy current probes, photoelectric position sensors and Hall effect sensors qualify as "zero speed sensors" because no relative movement is necessary for their operation.
Certain applications require zero speed sensing. One such application is wheel rotation sensing for anti-lock braking systems and traction control systems. Wheel rotation is a critical parameter in these applications and is an indication of brake lock-up or tire slippage.
FIG. 2 shows a graph of the output characteristics of a conventional eddy current probe sensing a ferrous toothed disc. It can be seen from the graph that as the probe stand-off spacing becomes greater, the signal differential between sensed teeth and valleys of the toothed disc is apparent.
In typical use, a stand-off spacing of about 1 millimeter (0.04 inches) probably is as close as one would want the probe to be located adjacent the toothed disc. Environmental considerations such as dirt, water and vibration tend to favor greater stand-off spacing, but the differential signal level (tooth to valley) approaches residual electronic noise levels with greater stand-off spacing. Also, any tendency of the sensor to move with respect to the disc causes the creation of interference signals, as would any out of roundness of the disc. Disc out of roundness or misalignment also causes a superimposed low frequency sinewave DC offset component on the output signal. Offset variations on the output signal can also occur from temperature induced drifts, normal aging of circuit components in addition to unwanted spacing variations, vibration and slow changes in circuit gain and offset.
The need for a solution to offset problems in eddy current detectors has existed for many years and has heretofore been unsolvable by eddy current sensor artisans. In the present invention, an eddy current detector detects an object with the compensation to compensate for variations including stand-off spacing, misalignment, temperature drift and aging of components.